<!DOCTYPE html>
<html style="font-size: 100px">
<head lang="en">
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0"/>
    <title></title>
    <script>
        var html = document.getElementsByTagName('html')[0];
        var width = window.innerWidth;
        var fontSize = 100/750*width;
        html.style.fontSize = fontSize +'px';
        window.onresize = function(){
            var html = document.getElementsByTagName('html')[0];
            var width = window.innerWidth;
            var fontSize = 100/750 * width;
            html.style.fontSize = fontSize +'px';
        }
    </script>
    <style>
        body,html{
            margin: 0;
            padding : 0;
        }
        div{
            width: 2rem;
            height: 2rem;
            background: red;
            color: #fff;
            font-size: 0.16rem;
        }
    </style>
</head>
<body>
    <div>AAA</div>
</body>
</html>